The present invention relates to oxygen sensors suitable for use in spacecraft and for other uses. More specifically it relates to oxygen sensors having improved useful life and stability in operation and suitable for use on the space shuttle.
It has been observed that prior art copper-alkalioxygen sensors exhibit a steady decline in oxygen sensitivity with time. Even the most expensive and sophisticated of these portable oxygen sensors must be adjusted continually over their useful life in order to compensate for and offset what has appeared to be a gradual reduction in sensing sensitivity.
It has also been observed that such prior art oxygen sensors have a total life of approximately 8000 hours when exposed to 1 atmosphere of air at 75.degree. F. Total life includes both useful operating life and exposure to oxygen for testing, storage and for any other reason. The calculated total life of a sensor as illustrated in FIG. 1 is approximately 9200 hours. The original specifications of the National Aeronautics and Space Administration for such sensors is 6236 operational hours of useful operating life.
During the time that such a conventional prior art sensor is functioning the sensitivity of the device, that is the voltage produced for a given rate of oxygen absorbed, continually decreases. Adjustment of the sensor is accordingly continually needed.
Because substantial testing and operation of sensors is necessary prior to an actual mission, an appreciable portion of the total life of such a sensor is used up in laboratory testing, in various calibrations and in an air exposure aboard the spacecraft before the scheduled launch. The delay problem can be further extended by delays in the actual launch thus using up a further portion of the total life of the sensor.
Calibration to adjust sensitivity is needed periodically.
Presently oxygen sensors having a total life of about 9000 hours exhibit a gradual decrease in sensitivity and this decrease amounts to approximately 0.5% per month. Such decrease is undesirable and necessitates frequent recalibration and adjustment of circuitry external to the sensor cell itself.